1. Field of the Invention
The present invention relates to a black level correction circuit for a television receiver or a television monitor.
2. Description of Background Information
An example of conventional black level correction circuit is explained with reference to FIG. I of the accompanying drawings.
In FIG. 1, an input terminal 1 receives a video signal which is demodulated by an RF tuner (not shown). This video signal is supplied to a clamp capacitor 2 and, through a node 3, to a level shift circuit 4 operating as an adder. An output signal of the level shift circuit 4 is supplied, through an amplifier 5, to a ORT section (not shown) including an RGB processing circuit and so on, and also to a switch circuit 6 and a clamp circuit 7. An output signal of the clamp circuit 7 is fed back to the node 8 through a switch circuit 8 to which a clamp pulse signal is supplied. The clamp circuit 7 and the switch circuit 8 together operate to adjust the pedestal level of the video signal to a normal level in such a manner as so compare the output signal of the level shift circuit 4 with a reference voltage Vc corresponding to the pedestal level during the presence of the clamp pulse signal which is supplied when the video signal is at its pedestal section, and to charge the clamp capacitor 2 (to supply electric charge to the clamp capacitor) in order that the output signal of the level shift circuit 4 is controlled at the pedestal level.
Blanking pulses are supplied to the switch circuit 6, so that it is opened during the period of sections of the video signal other than the luminance information section, such as the sync pulse section, so that only the luminance information of the video signal is supplied to a black peak detector circuit 9.
The black level detector circuit 9 compares the level of the supplied video signal with a level held by a black peak hold circuit 10, and supplies the video signal to the black peak hold circuit 10 when the level of the video signal is more closer to a maximum black level than to the held level. The black peak hold circuit 10 is made up of a capacitor and its charge and discharge circuit, and holds the supplied black peak voltage, and also changes the hold level gradually toward the white level in order to hold a new black peak level in response to changes in the picture information. This black peak level is supplied to a level comparator circuit 11. The level comparator circuit 11 compares the hold level with the maximum black level V.sub.B and supplies a difference signal between them to a voltage controlled variable gain amplifier (referred to simply as VOA hereinafter) 12 which is made up of a transistor-differential amplifier.
When the black peak level is at the maximum black level, the input signal of the VCA 12 becomes equal to 0, and VCA 12 produces no output signal. At a gain control input terminal of the VCA 12 there is supplied an output signal of a black detection circuit 13. Output signal of the VOA 12 is supplied to the level shift circuit 4 through a switch 15 which opens in response to blanking pulses, so as to transmit the signal only in the section of luminance signal. Thus, the output signal of the VCA 12 is superimposed on the output signal of the level shift circuit 4.
The video signal at the node 3 is supplied to the black detection circuit 13 through a switch circuit 16 which transmits only the luminance signal sections of the video signal in response to the blanking signal. The black detection circuit 13 detects portion(s) of the video signal whose level is lower than a predetermined threshold level V.sub.TH as a black signal, and supplies the level of the black signal exceeding the threshold level V.sub.TH (in the black direction) to the VOA 12 as its gain control signal. The gain of the VOA 12 is raised as the amplitude of the black signal increases.
With the above explained arrangement, when a black peak value of the video signal which exceeds the threshold level V.sub.TH is supplied to the terminal I, then it is compared with a previously held value by means of the black peak hold circuit 10. If the black level of the present time is higher than the previously held value, the held value of the black peak hold circuit is renewed, and the new held value is supplied to a VCA 14 through a level comparator circuit 13.
The output signal of the VCA 12 is superimposed on the black peaks of the video signal by means of the level shift circuit 4, and the peak value is further expanded toward the maximum black level. This operation of expanding the black level is a feedback control by which the peak level of the expanded black signal is controlled to be equal to the maximum black level V.sub.B. In addition, the black peak level being held varies gradually in the white direction as a result of discharging, as mentioned before. Therefore, as illustrated in FIG. 8 (A), the black peak of the video signal exceeding the threshold level V.sub.TH in the black direction is expanded toward the normal maximum black level, so that a black-corrected video signal is obtained and which in turn will be reproduced by a television receiver or a television monitor (not shown).
As shown in FIG. 8 (B), the video signal exceeding the threshold level V.sub.TH is transferred to the black-corrected video signal which is expanded in the black direction on the basis of its black peak level, at the VOA 12. Thus, there is an advantage that the reproduction of black in the television receiver and the like is improved.
On the other hand, if a video signal of entirely low luminance level, such as a scene of the sea, is supplied to this black level correction circuit, then the video signal will be expanded in the black direction entirely, because it stays below the threshold level. Therefore, it is undesirable that the reproduced picture looks too dark, or in other words, what is called a dark-saturation occurs. Furthermore, it is desired that the black level correction circuit is also effective for an entirely bright video signal which has little portion exceeding the threshold level V.sub.TH so that the contrast (contrast ratio) of the video signal is improved.